The present invention relates to a method and apparatus for reducing drag and vortex-induced-vibrations (‘VIV’) and, more particularly, reducing VIV and drag to cylindrical elements in marine environments.
Production of oil and gas from offshore fields has created many unique engineering challenges. One set of such challenges involves the use of cylindrical marine elements that are susceptible to large drag and vibrations when in the presence of significant ocean currents. Such marine elements are used in a variety of applications, including, e.g., subsea pipelines; drilling, production, import and export risers; tendons for tension leg platforms; legs for traditional fixed and for compliant platforms; space-frame members for platforms; cables; umbilicals; and other mooring elements for deepwater platforms; and, although not conventionally thought of as such, the hull and/or column structure for tension leg platforms (TLPs) or for spar type structures. These currents cause drag on the element and cause vortexes to shed from the sides, inducing drag forces and vibrations that can lead to the failure of the marine elements. Large drag forces can result in increased mooring or station keeping costs as well as the imposition of constraints on what kinds of systems are workable in a current environment (due to stress limitations, top angle limitation while drilling, etc.). Large vibrations (primarily vortex-induced vibrations) cause dynamic motions that, in turn, cause premature fatigue failures of structural members. In addition, large vibrations typically cause substantial increases in mean and dynamic drag forces. Finally, the presence of ocean currents can cause interference between adjacent structures.
One solution to the above set of issues is to use helical strakes. However, helical strakes are not very effective at reducing drag, and those are rarely used if drag reduction is important (e.g. drilling risers). Another solution which, if properly designed, can have all the positive benefits of helical strakes and can also reduce drag, is the use of fairings. However, there are many instances where the use of fairings is either impractical or uneconomical. An example is the reduction of drag and VIV for a drilling riser, where fairings can be very difficult to handle and therefore impose large usage costs in terms of lost time due to installation. For instance, fairings will not fit through a drilling rig rotary in order to allow installation above the rotary at a substantially reduced cost. Fairings also must typically be quite large and expensive to minimize drag coefficients. One of these challenges is dealing with effects of currents on fixed cylindrical marine elements.